1. Field of the Invention
The present invention relates to a gap detection device for detecting a gap between a nozzle of a laser beam machine for outputting a laser beam and an object to be machined by using the laser beam, a gap detection method and a laser beam machining system.
2. Description of the Background Art
As to laser beam machining, various techniques have been proposed. Japanese Patent Application Laid-Open No. 2000-234903, for example, discloses a technique to detect a gap between a nozzle of a laser beam machine for outputting a laser beam and a work which is an object to be machined. Japanese Patent Application Laid-Open No. 4-356391(1992) discloses a technique to ensure stabilization of static capacitance between a nozzle and a work by reducing plasma generated during laser beam machining.
In the above Japanese Patent Application Laid-Open No. 2000-234903, the frequency of an input signal to a sensor electrode is selected so that the impedance due to the plasma generated during laser beam machining should be a pure resistance and the pure resistance is eliminated from the composite impedance of the impedance due to the gap between the nozzle and the work and the impedance due to the plasma generated during the laser beam machining, to reduce the effect of the plasma on detection of the gap. Even if the frequency of the input signal to the sensor electrode is carefully selected, however, since the impedance due to the plasma slightly includes a static capacitance component as well as the resistance component, the technique of Japanese Patent Application Laid-Open No. 2000-234903 can not completely eliminate the effect of the plasma on detection of the gap. Therefore, it is difficult to detect the gap between the nozzle and the work with high accuracy.